﻿328 
  

  

  G. 
  ATTWOOD 
  ON 
  THE 
  GEOLOGY 
  

  

  33. 
  On 
  the 
  Geology 
  of 
  a 
  Part 
  of 
  Costa 
  Rica. 
  By 
  Geokge 
  Attwood, 
  

   Esq., 
  F.G.S., 
  F.C.S., 
  Assoc. 
  Mem. 
  Inst. 
  C.E., 
  Mem. 
  Am. 
  Inst, 
  

   M.E., 
  &c. 
  With 
  an 
  Appendix 
  by 
  W. 
  H. 
  Htjlleston, 
  Esq., 
  

   M.A., 
  F.G.S., 
  F.C.S., 
  &c. 
  (Head 
  May 
  24, 
  1882.) 
  

  

  [Plate 
  XII.] 
  

  

  Haying 
  spent 
  a 
  portion 
  of 
  last 
  winter 
  in 
  the 
  mountains 
  of 
  Costa 
  

   Rica, 
  I 
  was 
  enabled 
  to 
  collect 
  some 
  interesting 
  rocks, 
  and 
  also 
  to 
  

   compile 
  a 
  map 
  with 
  a 
  section 
  of 
  the 
  ground 
  over 
  which 
  I 
  travelled, 
  

   (PI. 
  XII.) 
  ; 
  and 
  I 
  now 
  beg 
  to 
  present 
  the 
  same 
  to 
  the 
  Society. 
  

  

  The 
  starting-point 
  of 
  my 
  travels 
  was 
  the 
  town 
  of 
  Punta 
  Arenas, 
  

   situated 
  on 
  the 
  Bay 
  of 
  Nicoya, 
  Republic 
  of 
  Costa 
  Rica, 
  Central 
  

   America. 
  The 
  town 
  is 
  built 
  near 
  the 
  extreme 
  end 
  of 
  a 
  peninsula 
  

   which 
  reaches 
  out 
  into 
  the 
  Bay 
  of 
  Nicoya 
  (fig. 
  1). 
  The 
  base 
  of 
  the 
  

   peninsula 
  is 
  a 
  calcareous 
  sandstone 
  (fig. 
  2), 
  containing 
  a 
  large 
  

   amount 
  of 
  carbonate 
  of 
  lime 
  and 
  some 
  mica-grains. 
  The 
  sandstone 
  

   is 
  nearly 
  horizontal 
  ; 
  and 
  where 
  it 
  has 
  been 
  exposed 
  by 
  the 
  action 
  

   of 
  the 
  sea 
  near 
  the 
  coast 
  it 
  shows 
  evidence 
  of 
  having 
  been 
  recently 
  

   attacked 
  by 
  boring 
  mollusca 
  (Lithoclomi). 
  The 
  peninsula, 
  which 
  is 
  

   almost 
  eight 
  miles 
  in 
  length, 
  and 
  from 
  half 
  a 
  mile 
  to 
  two 
  miles 
  

   in 
  width, 
  is 
  covered 
  with 
  a 
  dark 
  shining 
  sand, 
  which 
  varies 
  from 
  

   50 
  to 
  300 
  feet 
  in 
  thickness 
  and 
  extends 
  over 
  nearly 
  its 
  entire 
  area. 
  

   The 
  sand 
  is 
  fine, 
  the 
  largest 
  grains 
  or 
  crystals 
  not 
  exceeding 
  

   Jjy 
  inch 
  in 
  diameter, 
  while 
  the 
  average 
  diameter 
  is 
  about 
  -^ 
  inch. 
  

   When 
  examined 
  under 
  the 
  microscope, 
  it 
  contains 
  about 
  25 
  per 
  cent, 
  

   of 
  magnetite 
  and 
  about 
  25 
  per 
  cent, 
  of 
  quartz-grains, 
  the 
  remainder 
  

   being 
  made 
  up 
  of 
  felspar 
  and 
  probably 
  augite 
  and 
  their 
  decompo- 
  

   sition-products. 
  

  

  Pig. 
  1. 
  — 
  Longitudinal 
  Section 
  of 
  the 
  Peninsula 
  of 
  Punta 
  Arenas. 
  

  

  Punta 
  Arenas. 
  

  

  

  a. 
  Fine 
  Sand. 
  

  

  b. 
  Calcareous 
  Sandstone. 
  

  

  The 
  sand 
  deposit 
  appears 
  to 
  have 
  originated 
  from 
  the 
  material 
  

   brought 
  down 
  by 
  the 
  overflow 
  of 
  the 
  north-west 
  bank 
  of 
  the 
  Rio 
  

   Barranca. 
  The 
  Rio 
  Barranca 
  is 
  a 
  swift 
  river, 
  whose 
  source 
  is 
  in 
  the 
  

   hills 
  called 
  Cerro 
  el 
  Tigre, 
  distant 
  in 
  a 
  northerly 
  direction 
  about 
  

   thirty-five 
  miles, 
  and 
  at 
  an 
  elevation 
  of 
  over 
  3000 
  feet 
  above 
  the 
  

   sea-level. 
  There 
  is 
  no 
  evidence 
  of 
  a 
  late 
  overflow 
  ; 
  and, 
  from 
  the 
  high 
  

   bank 
  of 
  sand 
  left 
  by 
  the 
  original 
  overflow, 
  there 
  is 
  little 
  probability 
  

   of 
  such 
  a 
  thing 
  occurring 
  again. 
  

  

  The 
  gulf 
  of 
  Nicoya 
  is 
  being 
  rapidly 
  filled 
  up 
  to 
  the 
  north-west 
  of 
  

  

  